1. Field of the Invention
The present invention relates to a process and apparatus for the tubular blown film extrusion of a thermoplastic resin and more particularly and in a preferred embodiment, to the tubular blown film extrusion of a low strain hardening polymer, such as a low pressure-low density ethylene copolymer, employing an improved technique for cooling the blown tubular film.
2. Description of the Prior Art
In a conventional technique for forming tubular blown film suitable for the fabrication of bags and the like, a film-forming polymer, such as polyethylene, is extruded through an annular die arranged in an extrusion head so as to form a tube of molten polymer film having a smaller outer diameter than the intended diameter of the eventually-produced film tube. The molten film tube is withdrawn from the extruder and, after cooling to solidify the molten tube, is directed through flattening means such as a collapsing frame and a pair of rollers, which may be driven, and flattens the extruded film tube. Between the point of extrusion and the terminus of the flattening means, the film tube is inflated by means of air or some other gaseous medium to thereby form a film bubble. The bubble is maintained by pressurizing the gas trapped within the expanded film tube between the die and collapsing means. The driven nip rolls may withdraw the molten tubular film away from the annular die at a speed greater than the extrusion speed. This, together with the lateral expansion of the molten film bubble, decreases the film thickness and orients the blown film in both the machine and transverse directions. The degree of lateral expansion and the speed of the driven nip rolls may be controlled to provide the desired film thickness and orientation.
Cooling of the inflated molten tubing has heretofore been achieved by internal or external cooling of the film bubble or both. Regardless of the method of cooling, the point at which the molten film bubble solidifies is referred to in the art as the "frost line."
Internal bubble cooling may be provided by conventional means (see, e.g., U.S. Pat. No. 4,115,048). External cooling of the film bubble may be accomplished by providing one or more annular-shaped air rings around the film bubble. The prior art which teaches the use of air rings to cool a molten thermoplastic film bubble includes, for example, U.S. Pat. Nos. 3,867,083; 3,959,425; 3,976,732; 4,022,558; and 4,118,453; which all disclose the use of multiple, annular-shaped air rings disposed one above the other and around the film bubble. Means are provided both to blow air against the film bubble from each air ring and between adjacent air rings for the blown cooling air to exit from the system.
U.S. Pat. No. 3,548,042 discloses apparatus (and method) for cooling an extruded film bubble, comprising an annular-shaped air ring having an annular insert mounted therein such that cooling air blown into the air ring is divided into three components:
(1) a lowermost component directed perpendicularly against the film bubble just above the die orifice;
(2) an intermediate component which rises helically (clockwise) around and in contact with the film bubble; and
(3) an uppermost component which flows in a counter-clockwise direction in contact with the film bubble.
U.S. Pat. No. 3,568,252 discloses an annular device for cooling a film bubble which comprises separate cooling chambers, each provided with slits for blowing cooling air against the film bubble, and an inflating chamber between the slits maintained at a reduced pressure by virtue of the suction created by blowing cooling air from the upper slit. The reduced pressure may be controlled by means of valved air-inlet tubes communicating between the open air and the inflation chamber. Alternatively, separate cooling rings may be provided, each having cooling air slits, the lower cooling ring being provided with an inflation chamber in which a reduced pressure is maintained by virtue of the suction created by the cooling air exiting from the slit in the upper cooling ring. The separate cooling rings are separated by another ring to prevent heat radiation.
Other film bubble external cooling devices are disclosed in, for example, U.S. Pat. Nos. 3,888,609; 4,115,048; and No. Reissue 29,208.
Thermoplastic materials which may be formed into film by the tubular blown film process include polymers of olefins such as ethylene, propylene, and the like. Of these polymers, low density polyethylene (i.e., ethylene polymers having a density of about 0.94 g/cc and lower) constitutes the majority of film formed by the tubular blown film process. Conventionally, low density ethylene polymers have in the past been made commercially by the high pressure (i.e., at pressures of 15,000 psi and higher) homopolymerization of ethylene in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. Recently, low pressure processes for preparing low density ethylene polymers have been developed which have significant advantages as compared to the conventional high pressure process. One such low pressure process is disclosed in commonly-assigned, copending U.S. Applications Ser. No. 892,322, filed Mar. 31, 1978, now abandoned and Ser. No. 12,720, filed Feb. 16, 1979 (a foreign-filed application corresponding thereto has been published as European Patent Publication No. 4647).
The above-identified copending applications disclose a low pressure, gas phase process for producing low density ethylene copolymers having a wide density range of about 0.91 to about 0.94 g/cc and a melt flow ratio of from about 22 to about 36 and which have a relatively low residual catalyst content and a relatively high bulk density. The process comprises copolymerizing ethylene with one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons in the presence of a high activity magnesium-titanium complex catalyst prepared under specific activation conditions with an organo aluminum compound and impregnated in a porous inert carrier material. The copolymers (as applied to these polymers, the term "copolymers" as used herein is also meant to include polymers of ethylene with 2 or more comonomers) thus prepared are copolymers of predominantly (at least about 90 mole percent) ethylene and a minor portion (not more than 10 mole percent) of one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons which should not contain any branching on any of their carbon atoms which is closer than the fourth carbon atom. Examples of such alpha-olefin hydrocarbons are propylene, butene-1, hexene-1, 4-methyl pentene-1 and octene-1.
The catalyst may be prepared by first preparing a precursor composition from a titanium compound (e.g., TiCl.sub.4), a magnesium compound (e.g., MgCl.sub.2) and an electron donor compound (e.g., tetrahydrofuran) by, for example, dissolving the titanium and magnesium compounds in the electron donor compound and isolating the precursor by crystallization. A porous inert carrier (such as silica) is then impregnated with the precursor such as by dissolving the precursor in the electron donor compound, admixing the support with the dissolved precursor followed by drying to remove the solvent. The resulting impregnated support may be activated by treatment with an activator compound (e.g. triethyl aluminum).
The polymerization process may be conducted by contacting the monomers, in the gas phase, such as in a fluidized bed, with the activated catalyst at a temperature of about 30.degree. to 105.degree. C. and a low pressure of up to about 1000 psi (e.g., from about 150 to 350 psi).
The tubular blown film extrusion process may be employed to form a film from low pressure-low density ethylene copolymers. For example, a process for forming film from one such low pressure-low density ethylene copolymer is disclosed in commonly-assigned, copending U.S. Application Ser. No. 892,324, filed Mar. 31, 1978 now abandoned and U.S. Pat. No. 4,243,619, filed Feb. 16, 1979 (a foreign-filed application corresponding thereto has been published as European Patent Publication No. 6110). However, it has been found that the film production rates obtained in tubular film processes with low pressure-low density ethylene copolymers, utilizing conventional cooling devices and techniques, are low compared to the rates achievable in commercial tubular blown film processes using conventional high pressure-low density polyethylene. Specifically, the properties of low pressure-low density ethylene copolymers are such that commercially desirable high film production rates have not been achieved without film bubble instability. Stated conversely, film bubble instability problems prevent the commercially desirable high film production rates from being obtained in blown film extrusion processes with low pressure-low density ethylene copolymers. The reason for such failures, it is believed, is the extensional behavior of low pressure-low density ethylene copolymers. In comparison to conventional high pressure-low density polyethylene, certain low pressure-low density ethylene copolymers are softer and exhibit less melt strength in extension. As a result, when these low pressure-low density copolymers are extruded from the die in a tubular blown film processes and are externally cooled by blowing air against the resin, the film bubble is unable to resist deformation caused by the increased cooling required by increased throughput rates. In other words, film bubble instability results at higher throughput rates since such rates require more heat transfer in the cooling process which is usually accomplished by increasing the amount and/or velocity of cooling air which in turn deforms the film bubble due to the extensional behavior of low pressure-low density ethylene copolymers.